Earlier today, Apple stopped signing iOS 11.3.1 which meant that if you were on iOS 11.4, you could not downgrade back to the older version of the OS. The move from Apple came right when the Houdini iOS 11.3.1 semi-jailbreak was released.
If you are now ruining the opportunity lost and regretting not downgrading back to iOS 11.3.1 while the window was still open, here’s a simple trick using which you can still get your iPhone back to iOS 11.3. It has been discovered that Apple is still signing iOS 11.3 beta 5 and beta 6 release which means you can downgrade back to it on your iPhone running iOS 11.4. The exploit used in the iOS 11.3.1 jailbreak works on iOS 11.3 as well, so after downgrading back to the beta release of the OS, you can go ahead and jailbreak your iPhone.
The only downside of running the beta build on your iPhone would be a prompt to update it to a newer release that is available. But that’s a small price to pay if you are really keen on jailbreaking your iPhone by downgrading it back to iOS 11.3.
Its unclear as to how long Apple will continue signing the beta 5/6 release of iOS 11.3.1 so my recommendation would be to downgrade to it as soon as possible.